Endothelin (ET) was originally characterized as a potent vasoconstrictor produced by endothelial cells that regulates systemic blood pressures. ET has multiple effects on the kidney, including modulation of glomerular hemodynamics and electrolyte excretion. Recent studies have provided evidence that renal tubules synthesize ET. Additionally, nephron-derived ETs inhibit sodium and water absorption by renal tubule cells, raising the possibility of an autocrine function for these peptides. Four principal lines of investigations are planned: 1. Localization of the nephron sites of ET-1 and ET-3 production - Cultured cells derived from rabbit proximal tubule, thick ascending limb, cortical and inner medullary collecting duct will be analyzed for release of immunoreactive ET-1 and ET-3 and production of ET-1 and ET-3 MRNA. 2. Identification of the factors regulating nephron ET synthesis - The effects of the following factors on nephron ET release and MRNA production will be evaluated: a) agents known to regulate salt and water transport, including aldosterone, atrial natriuretic factor, and others; b) vasoactive compounds such as angiotensin and isoproterenol; and c) immunologic factors known to affect endothelial cell ET production, such as TGF-b, IL1, and TNF. 3. Characterization of nephron ET receptors - Each of the four cell types will be analyzed for ET-1 and ET-3 receptor number, binding affinity, and rates of association and dissociation. 4. Effects and mechanism of action of ET on sodium and water transport - ET-1 and ET-3 regulation of water transport mechanisms will be evaluated by studying the effect of these peptides on a) ADH-induced CAMP accumulation; b) prostaglandin E2 levels; c) inositol triphosphate and diacylglycerol levels; and d) intracellular calcium concentration. ET-1 and ET-3 regulation of sodium transport will be assessed by examining ouabain-inhabitable 86Rb uptake, a marker of Na/K ATPase activity. By pharmacologically controlling the activity of various components of the sodium transport pathway, these studies will identify the transport site that ET-1 and ET-3 affect. In summary, it is proposed that ET-1 and ET-3 are synthesized and released by several nephron segments. Furthermore, it is hypothesized that nephron-derived ET-1 and ET-3 can act locally, inhibiting sodium and water transport. Ultimately, this system may provide to play a significant role in the physiology and pathophysiology of renal salt and water excretion and in the pathogenesis of hypertension.